Is there a website where a list is maintained and constantly updated for undergrads/graduate students looking for internships/competitions/conferences in whatsoever field?
Such a website will be an invaluable resource!
Though it may be outdated now, my father and now myself are both part of AIESEC https://www.aiesec.org/
At the global field independent level the answer is no. At the national field specific level the answer is often no, but some countries have reasonable systems. In the UK jobs.ac.uk handles many academic jobs, but internships, competitors, and conferences are not particularly related.
I don't think there is one particular site that lists different internships, but you can defintiely look into joining listserves and organizations for you industry. For example, I work in international education and I joined NAFSA as well as their email listserv to learn more about industry trends, internships, and job openings. On an international level, you can check out websites like Go Overseas.com for a list of all international internship providers with reviews and website links.